Increase 3.0 Upgrade speed

I assume this is a rather low priority or maybe too much of a hassle to be worth the effort since I'm pretty sure i belong to a very small part of the userbase but:

After quite some time of usage i was assigned the task of upgrading our TeamPass 2.1.23 (you can laugh) instance to the latest version.

I'm in the middle of the jump from 2.1.27.36 to 3.0.21 and am waiting for the generating of sharekeys to finish right now.

You did warn the users that it can take quite a bit of time but i just figured i'd let you know in my particular case with 13 users and over 2100 items it takes roughly 20 minutes per user to generate those 2100 sharekeys. So this will take a few more hours of waiting while the server it's running on isn't particularly busy with the upgrade.

As a measurement to make the update process faster for future users i'd like to propose letting the upgrade wizard sending multiple POSTs to /install/upgrade_run_3.0.0_users.php simultanously to make use of more than a single core.

I haven't checked the code too much but given the fact that the workload i split into batches of 100 items per request to avoid timeouts there shouldn't be any worries regarding concurrency, right?